Substances in which both sensing and actuating capabilities are coupled by an intrinsic control mechanism are sometimes referred to as "smart" materials. For instance, a thin film of vanadium dioxide (VO.sub.2) deposited on a substrate is a classic example of a "smart" surface. VO.sub.2 undergoes a phase transition at a relatively low temperature. This phase transition is accompanied by extraordinary changes in the electronic and optical properties of VO.sub.2. In particulate or bulk form, VO.sub.2 undergoes a first-order monoclinic-to-tetragonal, and semiconducting-to-metallic phase transition, at temperatures of approximately 73.degree. C. to 68.degree. C. When a VO.sub.2 film is coated onto a transparent substrate, and illuminated so that the film absorbs sufficiently intense laser light, the resulting temperature increase can induce a rapid semi-conducting-to-metal phase transformation. The presence of the metallic VO.sub.2 phase then produces a reflecting surface, that subsequently strongly attenuates further transmission of the incident laser radiation through the coated substrate. Accordingly, the VO.sub.2 film performs both sensing and actuating functions through coupled intrinsic properties of the material.
Due to the reversible character of these changes, VO.sub.2 thin films have been investigated as active layers for a variety of applications. These applications include: self-protecting support windows, erasable optical data recording, thermal sensors, and coatings for energy-efficient windows.
In conventional thin-film systems, a continuous VO.sub.2 film is deposited on a surface of a substrate. As a result, thin layers of the VO.sub.2 on the surface of the substrate are exposed to environmental degradation and contamination. In addition, because of the two dimensional orientation of the film, cross talk or strain problems occur between a "switched" and "unswitched" region. Another difficulty occurs from the poor adhesion between the film and the substrate, which results in the degradation of optical properties at the interface of the film and the substrate.
Therefore, there is a need in the art for a multi-layer composite system consisting of an optically active phase such as VO.sub.2 and a durable host medium, which protects the three-dimensional orientation of optically active precipitates relative to the host medium, enhancing) the optical properties which can be obtained, including a well-defined variation in the optical reflectivity between "switched" and "unswitched" states.